As conventional techniques for manufacturing microfibers or nanofibers suitable for the production of artificial leather or the like, a sea-island type conjugated spinning technique, a dividing type conjugated spinning technique, a blend spinning technique, etc. are known.
However, in the sea-island type conjugated spinning technique or blend spinning technique, it is necessary to dissolve out and remove one of two polymer components of a fiber for making ultrafine fibers. And, in order to produce artificial leather from fibers manufactured by these techniques, complicated processes, such as melt spinning, fiber manufacturing, nonwoven fabric manufacturing, urethane impregnation and single-component dissolution, have to be performed. Nevertheless, it is impossible to manufacture a fiber with a diameter less than 1,000 nm by the two techniques.
Meanwhile, in the dividing type conjugated spinning technique, two polymer components (e.g., polyester and polyamide) with different dyeing properties co-exist within a fiber, thus dyeing stains appear and the artificial leather production process is complicated. Further, it was difficult to manufacture a fiber with a diameter less than 2,000 nm by the above method.
As another conventional technique for producing nanofibers, an electrostatic spinning method is proposed in U.S. Pat. No. 4,323,525. In the conventional electrostatic spinning technique, a polymer spinning solution in a spinning solution main tank is continuously supplied at a constant rate to a plurality of nozzles applied with a high voltage through a metering pump, and then the spinning solution supplied to the nozzles is spun and focused on a focusing device of endless belt type applied with a high voltage more than 5 kV, thereby producing a fibrous web. The produced fibrous web is needle-punched in the subsequent process, thus to manufacture a nonwoven fabric.
As described above, the conventional electrostatic spinning technique can manufacture a web and nonwoven fabric made up of nanofibers less than 1,000 nm. Therefore, in order to produce a continuous filament by the conventional electrostatic spinning technique, it is necessary to manufacture a monofilament by cutting a prepared nanofiber web to a predetermined length and then undergo a particular spinning process by blowing it again, which makes the process complicated.
In case of nonwoven fabric made up of nanofibers, there are restrictions in applying it in a wide range of various applications such as artificial leather due to the restrictions in the intrinsic properties of the nonwoven fabric. For reference, it is difficult for the nonwoven fabric made up of nanofibers to achieve properties of more than 10 MPa.
As a conventional technique for overcoming the conventional problems, Korean Patent Application No. 2004-6402 discloses a method for producing a continuous filament made up of nanofibers in which a ribbon-shaped nanofiber web of nanofibers is manufactured by electrostatically spinning a polymer spinning solution by a collector via nozzles, then a nanofiber filament of continuous filament type is produced by giving a twist to the nanofiber web while passing it through an air twisting machine, and then a continuous filament made up of nanofibers is produced by drawing the nanofiber filament.
In the aforementioned conventional method, however, electrostatically spun nanofibers cannot be oriented in the fiber axis direction, thus the focusability and the drawability are deteriorated, thereby deteriorating the mechanical properties of the produced continuous filament.
Moreover, the aforementioned conventional method is inconvenient in that in the event of using a narrow collector or a wide collector in order to manufacture a ribbon-shaped nanofiber web, a prepared nanofiber web has to be cut to a predetermined width.